1. Field of the Invention
The invention relates mainly to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition. The invention relates particularly to a liquid crystal composition having a positive dielectric anisotropy, and to a device of a twisted nematic (TN) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode or a polymer sustained alignment (PSA) mode containing the composition.
2. Related Art
In a liquid crystal display device, a classification based on the operation mode for liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), polymer sustained alignment (PSA), and so forth. A classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). The PM is further classified into static, multiplex and so forth, and the AM is further classified into a thin film transistor (TFT), a metal-insulator-metal (MIM) and so forth. The TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high-temperature type and a low-temperature type according to a production process. A classification based on a light source includes a reflection type utilizing natural light, a transmission type utilizing a backlight, and a semi-transmission type utilizing both the natural light and the backlight.
These devices contain a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to a temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying moving images with the device. Accordingly, a low viscosity of the composition is desirable. A low viscosity at a low temperature is more desirable.
TABLE 1General Characteristics of Liquid Crystal Composition and AMDeviceGeneral Characteristics of aGeneral Characteristics of an AMNo.CompositionDevice1Temperature range of aUsable temperature range is widenematic phase is wide2Viscosity is low1)Response time is short3Optical anisotropy isContrast ratio is highsuitable4Dielectric anisotropy isThreshold voltage is low and electricpositively or negativelypower consumption is lowlargeContrast ratio is high5Specific resistance is largeVoltage holding ratio is high and acontrast ratio is high6It is stable to ultravioletService life is longlight and heat1)A liquid crystal composition can be injected into a cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to maximize the contrast ratio. A suitable value of the product depends on the kind of operation modes. In a device having a TN mode or the like, a suitable value is approximately 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a low electric power consumption and a high contrast ratio. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a high contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance is desirable at room temperature and also at a high temperature after it has been used for a long time. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In the case where the stability is high, the device has a long service life. These characteristics are desirable for an AM device used in a liquid crystal projector, a liquid crystal television and so forth.
A composition having a positive dielectric anisotropy is used for an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having an IPS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having a PSA mode. Examples of the liquid crystal composition having a positive dielectric anisotropy are disclosed in the following patent documents No. 1 to 3. The dielectric anisotropy is not sufficiently large.    No. 1: WO 2006-038443 A; No. 2: JP 2008-038018 A; No. 3: WO 2008-102641 A.
A desirable AM device is characterized as having a wide usable temperature range, a short response time, a high contrast ratio, a low threshold voltage, a large voltage holding ratio, a long service life, and so forth. Even one millisecond shorter response time is desirable. Thus, a composition having characteristics such as a high maximum temperature of a nematic phase, a low minimum temperature of a nematic phase, a low viscosity, a large optical anisotropy, a large dielectric anisotropy, a small pretilt angle, a large specific resistance, a high stability to ultraviolet light, a high stability to heat, and so forth is especially desirable.